为什么此代码有效？

Question

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 work but I just found out that it does.

Answer 1

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 =)!

Answer 2

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

Answer 3

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.

Answer 4

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