The var retrieved from an array is undefined?

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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);

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

Undefined values with new Array() in JavaScript, var arr = Array(undefined, undefined, undefined, undefined, undefined, undefined); // Or `var arr = Array.apply(null, {length: 10}); unaligned with the real content, but useful for retrieve it and do some size condiderations. ARRAY FLOAT = Array[4] As a result, a variable set equal to !NULL can be thought of as an empty element in an array specification. Thinking of a null variable in this way makes it easy to add variables to an array without first checking to ensure that the array is defined.

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);

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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);
}

null, In APIs, null is often retrieved in a place where an object can be expected but no foo is known to exist now but it has no type or value: var foo = null; foo; //null When checking for null or undefined , beware of the differences between argument is not a valid code point · RangeError: invalid array length  ReturnIfAbrupt (items). if (arrayLike == null) { throw new TypeError('Array.from requires an array-like object - not null or undefined'); } // 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.

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Undefined values, A scalar variable (numeric or string) cannot have an 'undefined' value; if an Arrays and structures however can have an undefined state;  Undeclared: It occurs when we try to access any variable that is not initialized or declared earlier using var or const keyword. If we use ‘typeof’ operator to get the value of an undeclared variable, we will face the runtime error with return value as “undefined”. The scope of the undeclared variables is always global.

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Comments
  • Math.random is a function that needs to be called by (). Right now you're doing the equivalent of Function * 3), which is of course an invalid mathematical operation hence the result is NaN(Not a Number).
  • 2nd line of your code should be var randomNumber = Math.floor(Math.random() * 3);