Question

In the accepted answer on my earlier question
( What is the fastest way to generate a random integer in javascript? ), I was wondering how a number loses its decimals via the symbol |
.

For example:

var x = 5.12042;
x = x|0;

How does that floor the number to 5?

Some more examples:

console.log( 104.249834 | 0 ); //104
console.log( 9.999999 | 0 );   // 9

Answer 1

Because, according to the ECMAScript specifications, bitwise operators call ToInt32 on each expression to be evaluated.

See 11.10 Binary Bitwise Operators:

The production A : A @B, where @ is one of the bitwise operators in
the productions above, is evaluated as follows:

1. Evaluate A.

2. Call GetValue(Result(1)).

3. Evaluate B.

4. Call GetValue(Result(3)).

5. Call ToInt32(Result(2)).

6. Call ToInt32(Result(4)).

7. Apply the bitwise operator @ to Result(5) and Result(6). The result is a signed 32 bit integer.

8. Return Result(7).