JavaScript, using Math.floor to round a number

In JavaScript there are a couple of ways of rounding a number. The function Math.floor rounds the number down. It accepts a number n and returns the biggest integer that’s less than or equal n.

Base usage of Math.floor in JavaScript

With positive numbers it may seem as if we’re just throwing away a part of the number beyond the decimal dot.

console.log(Math.floor(7.25));   // 7
console.log(Math.floor(1.11));   // 1
console.log(Math.floor(0.99));   // 0

If you try to round the integer, it won’t change.

console.log(Math.floor(5));      // 5
console.log(Math.floor(100));    // 100

Negative numbers will be also rounded down. Not by the absolute value, though. So, you can’t just “cut by the decimal point” here.

console.log(Math.floor(-2.1));   // -3
console.log(Math.floor(-9.5));   // -10

Other data types

Technically, you can pass any value into Math.floor. A string, a boolean, an object or even a function.

JavaScript will try to convert the incoming argument into a number. If it succeeds, then everything will continue normally.

console.log(Math.floor("2.22")); // 2
console.log(Math.floor(true));   // 1
console.log(Math.floor("-1.5")); // -2

Otherwise, the result will be NaN — “not-a-number”.

console.log(Math.floor("hello"));                     // NaN
console.log(Math.floor({name: 'John', age: '25'}));   // NaN
console.log(Math.floor(console.log));                 // NaN

Exceptions

The values null and undefined behave differently. In the fist case you’ll get 0, and in the second - NaN.

console.log(Math.floor(null));      // 0   
console.log(Math.floor(undefined)); // NaN   

Remember this exception. It’s quite common on a Junior Technical Interview.

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