Implied string comparison, 0='', but 1='1'

Question

I was debugging something and discovered some strangeness in JavaScript:

alert(1=='') ==> false
alert(0=='') ==> true
alert(-1=='') ==> false

It would make sense that an implied string comparison that 0 should = '0'. This is true for all non-zero values, but why not for zero?

Answer 1

Javascript, like PHP, is weakly typed*. So when you compare 0 to '', the JS engine converts them to a similar datatype. Since 0 and '' both equate to boolean(false), "false == false" is true.

*Weakly typed languages do not require variables to be any specific data type, so you can set one variable as a string, change it to int, float, and back to string without the processor throwing errors.

Answer 2

This is just one of the truly hideous mangles that went into the JavaScript compromise. '' and 0 are both uninitialized values (equal to boolean false) and, therefore, equal.

To protect yourself from weird bugs like this, it's better to always use the === operator.

Answer 3

When javascript does implicit type conversions, the empty string literal will match the 0 integer. Do your comparison like this and you'll get your expected result:

alert(1==='') ==> false
alert(0==='') ==> false
alert(-1==='') ==> false

Answer 4

In many languages, the empty string can be coerced to false.

Beware of doing comparisons with == instead of ===:

alert('' == '0'); //false  
alert(0 == ''); // true  
alert(0 =='0'); // true

== is not transitive.

Answer 5

ECMA-262, 3rd edition, 11.9.3 regarding x == y, step 16:

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

The empty string '' gets converted to 0 before the comparison.

Answer 6

According to the Mozilla documentation on Javascript Comparison Operators

If the two operands are not of the same type, JavaScript converts the operands then applies strict comparison. If either operand is a number or a boolean, the operands are converted to numbers; if either operand is a string, the other one is converted to a string

What's actually happening is that the strings are being converted to numbers. For example:

1 == '1' becomes 1 == Number('1') becomes 1 == 1: true

Then try this one: 1 == '1.' becomes 1 == Number('1.') becomes 1 == 1: true If they were becoming strings, then you'd get '1' == '1.', which would be false.

It just so happens that Number('') == 0, therefore 0 == '' is true