为什么JS中的true =='true'语句返回false？

Question

Question is in the title. I've just tried to run next statements in Chrome console and got strange (as for me) result:

true == 'true' // returns false
'true' == true // returns false

Why does it go such way? Why doesn't typecast work there, but in the next statement works?

if ('true') true // returns true

Answer 1

Because they don't represent equally convertible types/values. The conversion used by == is much more complex than a simple toBoolean conversion used by if ('true') .

So given this code true == 'true' , it finds this:

"If Type(x) is Boolean , return the result of the comparison ToNumber(x) == y ."

So you see it starts by becoming ToNumber(true) == 'true' , which is 1 == 'true' , and then tries again, where it now finds:

If Type(x) is Number and Type(y) is String , return the result of the comparison x == ToNumber(y) .

So now it's doing 1 == ToNumber('true') , which is 1 == NaN , which of course is false .

Answer 2

The == operator uses ECMAScript's abstract equality algorithm which is quite complex. Its exact behavior depends on the types of each argument involved, and each step usually involves another invoking another ECMAScript function.

The if(condition) statement converts condition to a boolean using ECMAScript's ToBoolean which is simple enough to be expressed in a single table. As you can see in the spec, any string is truthy (according to ToBoolean ) if it has a nonzero length.

Answer 3

true = boolean type

'true' = string type

the expression "if ('true')" evaluates the 'true'(string) as true(boolean) on the same way that if('foo') or any other string does.

Answer 4

A non-empty string will return true:

• if ('0') true; // true
• if ('false') true; // true
• if ('anything') true; // true

A null string will return undefined and thus is falsy :

• if ('') true; // not true

When comparing types, JavaScript will try to do some magic for you:

• if (1 == "1") true; // true

But it fails when converting a string to boolean:

• if(true == "true") true; // not true

Answer 5

true is a boolean value 'true' is a string.

you are comparing different data types. look here: http://w3schools.com/js/js_datatypes.asp