Example
// Returns a random number:
Math.random();
JavaScript Math.random()
Math.random() returns a random number between 0 (inclusive), and 1 (exclusive):
Math.random() always returns a number lower than 1.
JavaScript Random Integers
Math.random() used with Math.floor() can be used to return random integers.
Note: There is no such thing as JavaScript integers. We are talking about numbers with no decimals here.
Example
// Return a random integer from 0 to 9 (both included):
Math.floor(Math.random() * 10);
Explained:
Math.random() returns a floating-point number between 0 (inclusive) and 1 (exclusive).
Example outputs: 0.0, 0.237, 0.9999, but never 1.
Math.random() * 10 gives a range from 0 up to but not including 10.
Example possible results: 0.0, 3.5, 9.99, etc.
Math.floor() rounds a number down to the nearest whole integer:
- 3.5 becomes 3
- 9.99 becomes 9
- 0.1 becomes 0
The possible integer results are then 0 through 9 (both inclusive).
In other words, the range is [0, 9].
Example
// Return a random integer from 0 to 10 (both included):
Math.floor(Math.random() * 11);
Example
// Return a random integer from 0 to 99 (both included):
Math.floor(Math.random() * 100);
Example
// Return a random integer from 0 to 100 (both included):
Math.floor(Math.random() * 101);
Example
// Return a random integer between 1 and 10 (both included):
Math.floor(Math.random() * 10) + 1;
Explained:
Math.random() returns a number from 0 (inclusive) up to but not including 1.
Multiplying by 10 gives a number from 0 up to but not including 10.
Adding 1 shifts that range to 1 up to but not including 11.
Math.floor() then rounds down, so you get an integer between 1 and 10.
Example
// Returns a random integer from 1 to 100 (both included):
Math.floor(Math.random() * 100) + 1;
Summary
| Col 1 | Col 2 | Col 3 |
|---|---|---|
| Expression | Range from | Range to |
| Math.random() | 0 | <1 |
| Math.random() * 10 | 0 | <10 |
| Math.random() * 100 | 0 | <100 |
| Math.floor(Math.random() * 10) | 0 | 9 |
A Proper Random Function
As you can see from the examples above, it might be a good idea to create a proper random function to use for all random integer purposes.
This JavaScript function always returns a random integer between min (included) and max (excluded):
Example
function getRndInteger(min, max) {
return Math.floor(Math.random() * (max - min) ) + min;
}
This JavaScript function always returns a random integer between min and max (both included):
Example
function getRndInteger(min, max) {
return Math.floor(Math.random() * (max - min + 1) ) + min;
}
