为什么空数组上的indexOf不返回-1？

Question

I am trying to check if an array contains a given value:

var arr = [];

if (arr.indexOf(2)) {
    console.log('yes');
}

Fiddle

Why is the if condition always met, although arr does not contain a value of 2 ? Shouldn't it return -1 and therefore nothing should be logged?

Answer 1

Your condition should be:

if (arr.indexOf(2) >= 0)

because in JavaScript, 0 is false when coalesced to a boolean, and any other number is considered true .

Therefore... -1 is coalesced to true .

 alert(!!-1) 

Answer 2

If you run this code in your browser's console, you'll see that it does return -1. However, in a JavaScript if statement, -1 is truthy so the alert will execute. As detailed in this excellent JavaScript equality table , there's only a very few values which will evaluate as false in an if statement: false , 0 , "" , null , undefined and NaN .

Answer 3

It does return -1 but -1 in Javascript is considered "truthy".

Try this to see that it is indeed returning -1:

var i = arr.indexOf(2);
console.log(i);

You need to explicitly compare for non-equality to -1 in your condition.

Here's an updated version of your jsfiddle:
http://jsfiddle.net/bmj8y8bj/