Why should you not use Number as a constructor?

javascript oop constructor jslint
GR1000 picture GR1000 · Dec 15, 2008 · Viewed 12.3k times · Source

I entered this statement in JSLint:

var number = new Number(3);

And received the following message:

Do not use Number as a constructor.

Why is that? The statement is creating a number object, not a primitive value, so I don't see why using new is a problem.

EDIT: Thanks for all the responses. They've got me thinking further, so I posted a follow-up question here.

Answer

Matthew Crumley picture Matthew Crumley · Dec 15, 2008

In addition to breaking === and typeof returning "object", using the Number constructor also changes the way the value is used in boolean contexts. Since "new Number(0)" is an object, not a literal value, it evaluates as "true" because it is not null. So for example:

var n1 = 0;
var n2 = new Number(0);

n1 == n2  // true
n1 === n2 // false
if (n1) {
    // Doesn't execute
}
if (n2) {
    // Does execute, because n2 is an object that is not null
}

Edit: Even worse than breaking === between number literals and Number objects, == doesn't even work between two Number objects (at least not in the intuitive way -- they test for identity, not equality).

var n1 = new Number(3);
var n2 = new Number(3);

alert(n1 == n2); // false
alert(n1 === n2); // false

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