Math.random Examples

The general syntax for this method is:

Math.random()

This method returns a random decimal value between 0 and 1, which can be stored in a variable or used in a calculation.

Note that 0 is a possible selection, but 1 is NOT.

Example

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for (i=0; i<5; i++){
   let randNum = Math.random();
   console.log(randNum);
}

Console Output

0.34992592600591066
0.11861535165960668
0.019710093901842862
0.7751799992655235
0.46782849511194136

Generate a Random Integer

If a random integer must be generated, the result of Math.random() can be manipulated with operators (+, -, *, /) and other Math methods.

The trick to creating a random integer is to multiply Math.random() by a whole number and then round the result to remove the decimal portion. The choice of using the ceil, floor, or round method affects the numbers generated.

Explore the example below:

Example

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for (i=0; i < 5; i++){
   let num = Math.random()*10;
   console.log(`floor = ${Math.floor(num)}, ceil = ${Math.ceil(num)}, round = ${Math.round(num)}`);
}

Console Output

floor = 0, ceil = 1, round = 0
floor = 6, ceil = 7, round = 7
floor = 2, ceil = 3, round = 3
floor = 8, ceil = 9, round = 8
floor = 9, ceil = 10, round = 10

After multiplying Math.random() by 10, applying the floor method gives numbers between 0 and 9. Using the ceil method shifts the range up one digit, generating values between 1 and 10. Using the round method gives the widest range, generating numbers between 0 and 10.

Rather than trying to remember which method to use, one choice is to ALWAYS use floor to round to an integer:

  1. Math.floor(Math.random()*10) generates a number from 0 - 9.
  2. Math.floor(Math.random()*120) generates a number from 0 - 119.

To start our range at 1, just add 1 to the rounded value:

  1. Math.floor(Math.random()*10) + 1 generates a number from 1 - 10.
  2. Math.floor(Math.random()*120) + 1 generates a number from 1 - 120.

By changing the value that multiplies Math.random() we specify the range for the numbers we want to generate.

  1. Math.floor(Math.random()*maxValue) generates a number from 0 to (maxValue-1).
  2. Math.floor(Math.random()*maxValue) + 1 generates a number from 1 to maxValue.

Try It

Explore generating random numbers at this repl.it.

  1. Generate random integers from 1 - 100.
  2. Generate random integers from -5 - 0, but NOT including 0.
  3. Challenge: Generate random integers from 20 - 30.