JavaScript - why is Math.max() returning NaN on array of integers?

Your code doesn't work because Math.max() expects each argument to be a valid number.

From the documentation:
The Math.max() returns NaN if any parameter isn't a number and can't be converted into one.

What you are trying to do is provide one argument that is an array, not a number.

Suggested solutions

1. Use spread syntax (...)

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var numbers = [1, 4, 2, 5, 3];

​

console.log(Math.max(...numbers)); // 5

Note:

The spread syntax was introduced in ES6.

2. Manually specify each argument without using an array

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console.log(Math.max(1, 4, 2, 5, 3)); // 5

3. Explicitly call Math.max() by using apply() method and passing an array of arguments

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var numbers = [1, 4, 2, 5, 3];

​

console.log(Math.max.apply(Math, numbers)); // 5

See also

1. JavaScript - ECMAScript / ES versions and features

References

1. Spread syntax (...) - JavaScript | MDN
2. Function.prototype.apply() - JavaScript | MDN