从数组检索的var是不确定的？

Question

The following code includes an array called buttonColours, a random number of 0 to 3, then the randomColour is supposed to be a random colour in the array of buttonColours, but it doesn't. It classifies randomColour as undefined.

 var buttonColours = ["blue", "red", "green"]; var randomNumber = Math.floor(Math.random * 3); var randomColour = buttonColours[randomNumber]; console.log(randomColour); 

The console.log only logs randomColor as undefined. Is there a reason why?

Answer 1

You'll know directly the problem if you did a console.log on each line :

 var buttonColours = ["blue", "red", "green"]; console.log(buttonColours); // ["blue", "red", "green"] var randomNumber = Math.floor(Math.random * 3); console.log(randomNumber); // NaN var randomColour = buttonColours[randomNumber]; console.log(randomColour); // undefined 

The problem is your Math.floor(Math.random * 3) is returning NaN, as it should be Math.random()

 var buttonColours = ["blue", "red", "green"]; var randomNumber = Math.floor(Math.random() * 3); var randomColour = buttonColours[randomNumber]; console.log(randomColour); 

Answer 2

I think this is what you want to achieve.

If you will try to find out the result of each line you will come to know that Math.floor(Math.random * 3) is returning 'Nan' so the finally when you try to select the element of that position will be undefined .

This is returning Nan Because Math.random is a function and you need () to call a function. As you can see in my code it is Math.random()

 var buttonColours = ["blue", "red", "green"]; var randomColour = buttonColours[Math.floor(Math.random() * buttonColours.length)]; console.log(randomColour); 

Answer 3

You can get a random number within a range by this function. The Math.random() return a random value within [0,1). So you have to map the [0,1) with the range you given.

function getRandom(min, max) {
  return Math.floor(Math.random() * (max-min) + min);
}

See your solution in this snippet-

 var buttonColours = ["blue", "red", "green"]; var randomNumber = getRandom(0, buttonColours.length); var randomColour = buttonColours[randomNumber]; console.log(randomColour); function getRandom(min, max) { return Math.floor(Math.random() * (max-min) + min); } 

Answer 4

The problem is here:

var randomNumber = Math.floor(Math.random * 3);

Math.random() is a method, not a property, of the Math object, hence it needs to be called . Calling is done by following the name of the function with a pair of parentheses, so to call a function named sayHello, I would do:

sayHello()

So to fix your problem, place a pair of parentheses after Math.random, so that line looks like so:

var randomNumber = Math.floo(Math.random() * 3);

If you want to know what Math.random() actually does, look at MDN's documentation for it :

The Math.random() function returns a floating-point, pseudo-random number in the range 0–1 (inclusive of 0, but not 1)...

So essentially it returns a random number from 0 to 0.9999999..., which is why your line of code:

Math.floor(Math.random() * 3)

Returns an integer 0, 1 or 2, but not 3.