Check if a number has a decimal place/is a whole number

Using modulus will work:

num % 1 != 0
// 23 % 1 = 0
// 23.5 % 1 = 0.5

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Note that this is based on the numerical value of the number, regardless of format. It treats numerical strings containing whole numbers with a fixed decimal point the same as integers:

'10.0' % 1; // returns 0
10 % 1; // returns 0
'10.5' % 1; // returns 0.5
10.5 % 1; // returns 0.5

Or you could just use this to find out if it is NOT a decimal:

string.indexOf(".") == -1;

Number.isInteger(23);  // true
Number.isInteger(1.5); // false
Number.isInteger("x"); // false: 

Number.isInteger() is part of the ES6 standard and not supported in IE11.

It returns false for NaN, Infinity and non-numeric arguments while x % 1 != 0 returns true.