Why does this code work?

I thought if statements weren't supposed to contain assignment operators but rather the comparison operators (==, ===) but this works perfectly. Why?

var foo = true,
    bar = true;
if (foo = true) {
    console.log('foo is true');
}

I was taught that this wouldn't wo开发者_开发知识库rk but I just found out that it does.

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What you're actually doing, is still comparing:

if ((foo = true) == true) ...

This is an 'abbreviation' for:

foo = true;
if (foo == true) ...

So it does make sense =)!

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From the ES5.1 specification (12.5)

IfStatement :
    if ( Expression ) Statement  else Statement
    if ( Expression ) Statement

Any valid expression can be placed inside an if.

foo = true is an expression and it evaluates to true.

To avoid bugs like writing = instead of == in the future write it like

if (true = foo) {

}

Which will throw a assignment error since you can't assign values to literal values like true

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The assignment is evaluated to true, beacuse JavaScript returns the value that it sets the variable to.

var foo = true,
    bar = true;
if (foo = true) {
    console.log('foo is true');
}

becomes:

var foo = true,
    bar = true;
if (true) {
    console.log('foo is true');
}

which passes the if. Note that setting to false would not work, because the conditional would evaluate to false which does not pass the if.

Specification about if:

if ( Expression ) Statement

You are using the assignment expression:

AssignmentExpression : ConditionalExpression LeftHandSideExpression AssignmentOperator AssignmentExpression

The = is specified as:

Simple Assignment ( = )

The production AssignmentExpression : LeftHandSideExpression = AssignmentExpression is evaluated as follows:

...

2 . Let rref be the result of evaluating AssignmentExpression.

3 . Let rval be GetValue(rref).

...

6. Return rval.

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Tragically, you are in fact assigning foo to true. :)