Functions Cant Contain Non-Blocking Assignments Images

JavaScript supports a compact set of statements, specifically control flow statements, that you can use to incorporate a great deal of interactivity in your application. This chapter provides an overview of these statements.

The JavaScript reference contains exhaustive details about the statements in this chapter. The semicolon () character is used to separate statements in JavaScript code.

Any JavaScript expression is also a statement. See Expressions and operators for complete information about expressions.

Block statement

The most basic statement is a block statement that is used to group statements. The block is delimited by a pair of curly brackets:

{ statement_1; statement_2; . . . statement_n; }


Block statements are commonly used with control flow statements (e.g. , , ).

while (x < 10) { x++; }

Here, is the block statement.

Important: JavaScript prior to ECMAScript2015 does not have block scope. Variables introduced within a block are scoped to the containing function or script, and the effects of setting them persist beyond the block itself. In other words, block statements do not define a scope. "Standalone" blocks in JavaScript can produce completely different results from what they would produce in C or Java. For example:

var x = 1; { var x = 2; } console.log(x); // outputs 2

This outputs 2 because the statement within the block is in the same scope as the statement before the block. In C or Java, the equivalent code would have outputted 1.

Starting with ECMAScript2015, the variable declaration is block scoped. See the reference page for more information.

Conditional statements

A conditional statement is a set of commands that executes if a specified condition is true. JavaScript supports two conditional statements: and .


Use the statement to execute a statement if a logical condition is true. Use the optional clause to execute a statement if the condition is false. An statement looks as follows:

if (condition) { statement_1; } else { statement_2; }

Here the  can be any expression that evaluates to true or false. See Boolean for an explanation of what evaluates to and . If evaluates to true, is executed; otherwise, is executed. and can be any statement, including further nested statements.

You may also compound the statements using to have multiple conditions tested in sequence, as follows:

if (condition_1) { statement_1; } else if (condition_2) {   statement_2; } else if (condition_n) {   statement_n; } else {   statement_last; } 

In the case of multiple conditions only the first logical condition which evaluates to true will be executed. To execute multiple statements, group them within a block statement () . In general, it's good practice to always use block statements, especially when nesting statements:

if (condition) { statement_1_runs_if_condition_is_true; statement_2_runs_if_condition_is_true; } else { statement_3_runs_if_condition_is_false; statement_4_runs_if_condition_is_false; }

It is advisable to not use simple assignments in a conditional expression, because the assignment can be confused with equality when glancing over the code. For example, do not use the following code:

if (x = y) { /* statements here */ }

If you need to use an assignment in a conditional expression, a common practice is to put additional parentheses around the assignment. For example:

if ((x = y)) { /* statements here */ }

Falsy values

The following values evaluate to false (also known as Falsy values):

  • the empty string ()

All other values, including all objects, evaluate to true when passed to a conditional statement.

Do not confuse the primitive boolean values and with the true and false values of the object. For example:

var b = new Boolean(false); if (b) // this condition evaluates to true if (b == true) // this condition evaluates to false


In the following example, the function returns if the number of characters in a object is three; otherwise, it displays an alert and returns .

function checkData() { if (document.form1.threeChar.value.length == 3) { return true; } else { alert('Enter exactly three characters. ' + document.form1.threeChar.value + ' is not valid.'); return false; } }


A statement allows a program to evaluate an expression and attempt to match the expression's value to a case label. If a match is found, the program executes the associated statement. A statement looks as follows:

switch (expression) { case label_1: statements_1 [break;] case label_2: statements_2 [break;] ... default: statements_def [break;] }

The program first looks for a clause with a label matching the value of expression and then transfers control to that clause, executing the associated statements. If no matching label is found, the program looks for the optional clause, and if found, transfers control to that clause, executing the associated statements. If no clause is found, the program continues execution at the statement following the end of . By convention, the clause is the last clause, but it does not need to be so.

The optional statement associated with each clause ensures that the program breaks out of once the matched statement is executed and continues execution at the statement following switch. If is omitted, the program continues execution at the next statement in the statement.


In the following example, if evaluates to "Bananas", the program matches the value with case "Bananas" and executes the associated statement. When is encountered, the program terminates and executes the statement following . If were omitted, the statement for case "Cherries" would also be executed.

switch (fruittype) { case 'Oranges': console.log('Oranges are $0.59 a pound.'); break; case 'Apples': console.log('Apples are $0.32 a pound.'); break; case 'Bananas': console.log('Bananas are $0.48 a pound.'); break; case 'Cherries': console.log('Cherries are $3.00 a pound.'); break; case 'Mangoes': console.log('Mangoes are $0.56 a pound.'); break; case 'Papayas': console.log('Mangoes and papayas are $2.79 a pound.'); break; default: console.log('Sorry, we are out of ' + fruittype + '.'); } console.log("Is there anything else you'd like?");

Exception handling statements

You can throw exceptions using the statement and handle them using the statements.

Exception types

Just about any object can be thrown in JavaScript. Nevertheless, not all thrown objects are created equal. While it is fairly common to throw numbers or strings as errors it is frequently more effective to use one of the exception types specifically created for this purpose:


Use the statement to throw an exception. When you throw an exception, you specify the expression containing the value to be thrown:

throw expression;

You may throw any expression, not just expressions of a specific type. The following code throws several exceptions of varying types:

throw 'Error2'; // String type throw 42; // Number type throw true; // Boolean type throw {toString: function() { return "I'm an object!"; } };

Note: You can specify an object when you throw an exception. You can then reference the object's properties in the block.

// Create an object type UserException function UserException(message) { this.message = message; = 'UserException'; } // Make the exception convert to a pretty string when used as a string // (e.g. by the error console) UserException.prototype.toString = function() { return + ': "' + this.message + '"'; } // Create an instance of the object type and throw it throw new UserException('Value too high');


The statement marks a block of statements to try, and specifies one or more responses should an exception be thrown. If an exception is thrown, the statement catches it.

The statement consists of a block, which contains one or more statements, and a block, containing statements that specify what to do if an exception is thrown in the block. That is, you want the block to succeed, and if it does not succeed, you want control to pass to the block. If any statement within the block (or in a function called from within the block) throws an exception, control immediately shifts to the block. If no exception is thrown in the block, the block is skipped. The block executes after the and blocks execute but before the statements following the statement.

The following example uses a statement. The example calls a function that retrieves a month name from an array based on the value passed to the function. If the value does not correspond to a month number (1-12), an exception is thrown with the value and the statements in the block set the variable to .

function getMonthName(mo) { mo = mo - 1; // Adjust month number for array index (1 = Jan, 12 = Dec) var months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']; if (months[mo]) { return months[mo]; } else { throw 'InvalidMonthNo'; //throw keyword is used here } } try { // statements to try monthName = getMonthName(myMonth); // function could throw exception } catch (e) { monthName = 'unknown'; logMyErrors(e); // pass exception object to error handler -> your own function }

The block

You can use a block to handle all exceptions that may be generated in the block.

catch (catchID) { statements }

The block specifies an identifier ( in the preceding syntax) that holds the value specified by the statement; you can use this identifier to get information about the exception that was thrown. JavaScript creates this identifier when the block is entered; the identifier lasts only for the duration of the block; after the block finishes executing, the identifier is no longer available.

For example, the following code throws an exception. When the exception occurs, control transfers to the block.

try { throw 'myException'; // generates an exception } catch (e) { // statements to handle any exceptions logMyErrors(e); // pass exception object to error handler }

The block

The block contains statements to execute after the and blocks execute but before the statements following the statement. The block executes whether or not an exception is thrown. If an exception is thrown, the statements in the block execute even if no block handles the exception.

You can use the block to make your script fail gracefully when an exception occurs; for example, you may need to release a resource that your script has tied up. The following example opens a file and then executes statements that use the file (server-side JavaScript allows you to access files). If an exception is thrown while the file is open, the block closes the file before the script fails.

openMyFile(); try { writeMyFile(theData); //This may throw an error } catch(e) { handleError(e); // If we got an error we handle it } finally { closeMyFile(); // always close the resource }

If the block returns a value, this value becomes the return value of the entire production, regardless of any statements in the and blocks:

function f() { try { console.log(0); throw 'bogus'; } catch(e) { console.log(1); return true; // this return statement is suspended // until finally block has completed console.log(2); // not reachable } finally { console.log(3); return false; // overwrites the previous "return" console.log(4); // not reachable } // "return false" is executed now console.log(5); // not reachable } f(); // console 0, 1, 3; returns false

Overwriting of return values by the block also applies to exceptions thrown or re-thrown inside of the block:

function f() { try { throw 'bogus'; } catch(e) { console.log('caught inner "bogus"'); throw e; // this throw statement is suspended until // finally block has completed } finally { return false; // overwrites the previous "throw" } // "return false" is executed now } try { f(); } catch(e) { // this is never reached because the throw inside // the catch is overwritten // by the return in finally console.log('caught outer "bogus"'); } // OUTPUT // caught inner "bogus"

Nesting try...catch statements

You can nest one or more statements. If an inner statement does not have a block, it needs to have a block and the enclosing statement's block is checked for a match. For more information, see nested try-blocks on the reference page.

Utilizing objects

Depending on the type of error, you may be able to use the 'name' and 'message' properties to get a more refined message. 'name' provides the general class of Error (e.g., 'DOMException' or 'Error'), while 'message' generally provides a more succinct message than one would get by converting the error object to a string.

If you are throwing your own exceptions, in order to take advantage of these properties (such as if your catch block doesn't discriminate between your own exceptions and system ones), you can use the Error constructor. For example:

function doSomethingErrorProne() { if (ourCodeMakesAMistake()) { throw (new Error('The message')); } else { doSomethingToGetAJavascriptError(); } } .... try { doSomethingErrorProne(); } catch (e) { console.log(; // logs 'Error' console.log(e.message); // logs 'The message' or a JavaScript error message) }


Starting with ECMAScript2015, JavaScript gains support for objects allowing you to control the flow of deferred and asynchronous operations.

A is in one of these states:

  • pending: initial state, not fulfilled or rejected.
  • fulfilled: successful operation
  • rejected: failed operation.
  • settled: the Promise is either fulfilled or rejected, but not pending.

Loading an image with XHR

A simple example using and to load an image is available at the MDN GitHub js-examples repository. You can also see it in action. Each step is commented and allows you to follow the Promise and XHR architecture closely. Here is the uncommented version, showing the flow so that you can get an idea:

function imgLoad(url) { return new Promise(function(resolve, reject) { var request = new XMLHttpRequest();'GET', url); request.responseType = 'blob'; request.onload = function() { if (request.status === 200) { resolve(request.response); } else { reject(Error('Image didn\'t load successfully; error code:' + request.statusText)); } }; request.onerror = function() { reject(Error('There was a network error.')); }; request.send(); }); }

For more detailed information, see the reference page and the Using Promises guide.

As I was working on a SystemVerilog based FPGA design, I came across a situation where I had to compute the sum of an array of 4 elements on a clock edge. I was able to do that using a for loop with non blocking assign statements.

The design synthesized successfully on Quartus 15.0 but when I tried to run the simulations using the same RTL on Modelsim Altera the results were unexpected. I have written a sample code to illustrate the same.

This image shows the simulation results.

In this sample code, sum is computed using non-blocking assignments. It was intended to have a value of (1+3+5+6)=15 on the first posedge of clk; which I have observed in original hardware. But in simulation the result was 6 at the posedge of clk(which is abc[3]). Since the systemverilog simulators schedule the assignments for non-blocking statements, I believe 4 instances of sum are created viz.

As all the scheduled assignments happen simultaneously, may be the last instance updated sum and we have a value of sum <= 0 + 6. Please correct me if I'm wrong.

Now my question is how do I make the simulator schedule these assignments sequentially so that I get a value of 15 even in simulation? Since blocking assignments are not an option in synthesis, I cant find any way to keep the RTL consistent. Thanks.



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