What does "i = +i" mean in Javascript?

Question

In jquery source:

eq: function( i ) {
    i = +i;
    return i === -1 ?
        this.slice( i ) :
        this.slice( i, i + 1 );
},

Is it used for make sure parse i to int?

Answer 1

Is it used for make sure parse i to int?

No, it is to make sure that i is a number (either float or int). Given what the function is doing, it was better to convert i to an non-decimal value though, I'm not sure how slice handles decimals.

More information: MDN - Arithmetic Operators

Answer 2

Almost, but any number is fine.

[ECMA-262: 11.4.6]: The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

1. Let expr be the result of evaluating UnaryExpression.
2. Return ToNumber(GetValue(expr)).