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JavaScript: Initializes an array containing the numbers in the specified range (in reverse) where start and end are inclusive with their common difference step

JavaScript fundamental (ES6 Syntax): Exercise-214 with Solution

Write a JavaScript program to initializes an array containing the numbers in the specified range (in reverse) where start and end are inclusive with their common difference step.

  • Use Array.from(Math.ceil((end+1-start)/step)) to create an array of the desired length(the amounts of elements is equal to (end-start)/step or (end+1-start)/step for inclusive end), Array.prototype.map() to fill with the desired values in a range.
  • Omit the second argument, start, to use a default value of 0.
  • Omit the last argument, step, to use a default value of 1.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const initializeArrayWithRangeRight = (end, start = 0, step = 1) =>
  Array.from({ length: Math.ceil((end + 1 - start) / step) }).map(
    (v, i, arr) => (arr.length - i - 1) * step + start
  );

console.log(initializeArrayWithRangeRight(5));
console.log(initializeArrayWithRangeRight(7, 3));
console.log(initializeArrayWithRangeRight(9, 0, 2));

Sample Output:

[5,4,3,2,1,0]
[7,6,5,4,3]
[8,6,4,2,0]

Pictorial Presentation:

JavaScript Fundamental: Initializes an array containing the numbers in the specified range (in reverse) where start and end are inclusive with their common difference step.
JavaScript Fundamental: Initializes an array containing the numbers in the specified range (in reverse) where start and end are inclusive with their common difference step.

Flowchart:

flowchart: Initializes an array containing the numbers in the specified range (in reverse) where start and end are inclusive with their common difference step.

Live Demo:

See the Pen javascript-fundamental-exercise-214 by w3resource (@w3resource) on CodePen.


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JavaScript: Tips of the Day

Checking if a key exists in a JavaScript object?

Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?

var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!

You should instead use the in operator:

"key" in obj // true, regardless of the actual value

If you want to check if a key doesn't exist, remember to use parenthesis:

!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"

Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:

obj.hasOwnProperty("key") // true

Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined? var obj = { key: undefined }; obj["key"] !== undefined // false, but the key exists! You should instead use the in operator: "key" in obj // true, regardless of the actual value If you want to check if a key doesn't exist, remember to use parenthesis: !("key" in obj) // true if "key" doesn't exist in object !"key" in obj // ERROR! Equivalent to "false in obj" Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty: obj.hasOwnProperty("key") // true For performance comparison between the methods that are in, hasOwnProperty and key is undefined.

Ref: https://bit.ly/2CFNp1X