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

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