JavaScript 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.
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);
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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);
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Example
// Return a random integer from 0 to 99 (both included):
Math.floor(Math.random() * 100);
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Example
// Return a random integer from 0 to 100 (both included):
Math.floor(Math.random() * 101);
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Example
// Return a random integer between 1 and 10 (both included):
Math.floor(Math.random() * 10) + 1;
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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;
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Summary
| 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;
}
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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;
}
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