How to write isNumber() in JavaScript? [duplicate]

My guess would be:

function isNumber(val) { return val === +val;
}

Is there a better way?

Previous Reference

Validate decimal numbers in JavaScript - IsNumeric()

8

6 Answers

var isNumber = function isNumber(value)
{ return typeof value === 'number' && isFinite(value);
}

This code above is taken from the book "JavaScript - the good parts".

/* ANSWER UPDATE BEGIN - in reply to some comments
below from the person who asked the question and others */

var isNumber = function isNumber(value) { return typeof value === 'number' && isFinite(value);
}
var isNumberObject = function isNumberObject(n) { return (Object.prototype.toString.apply(n) === '[object Number]');
}
var isCustomNumber = function isCustomNumber(n){ return isNumber(n) || isNumberObject(n);
}
console.log(isCustomNumber(new Number(5)));
console.log(isCustomNumber(new Number(5.2)));
console.log(isCustomNumber(new Number(5.5)));
console.log(isCustomNumber(new Number(-1)));
console.log(isCustomNumber(new Number(-1.5)));
console.log(isCustomNumber(new Number(-0.0)));
console.log(isCustomNumber(new Number(0.0)));
console.log(isCustomNumber(new Number(0)));
console.log(isCustomNumber(new Number(1e5)));
console.log(isCustomNumber(5));
console.log(isCustomNumber(5.2));
console.log(isCustomNumber(5.5));
console.log(isCustomNumber(-1));
console.log(isCustomNumber(-1.5));
console.log(isCustomNumber(-0.0));
console.log(isCustomNumber(0.0));
console.log(isCustomNumber(0));
console.log(isCustomNumber(1e5));

/* ANSWER UPDATE END - in reply to some comments
below from the person who asked the question and others */

12

If you do not want to include strings, and you do want to include Infinity, you can compare the Number coercion of the argument to the argument:

function isNumber(n){ return Number(n)=== n;
}

//test

[0, 1, 2, -1, 1.345e+17, Infinity, false, true, NaN, '1', '0'].map(function(itm){ return itm+'= '+isNumber(itm);
});
// returned values
0= true
1= true
2= true
-1= true
134500000000000000= true
Infinity= true
false= false
true= false
NaN= false
'1'= false
'0'= false

1

function isNumber(val){ return typeof val==='number';
}

2

If you want "23" to be a number, then

function isNumber(val) { return !isNaN(val);
}

2

function isNumber(val) { // negative or positive return /^[-]?\d+$/.test(val); }

3

I like this solution:

function isNumber(val) { return (val >=0 || val < 0);
}

3