How to initialize an array’s length in JavaScript?

by

  • Array(5) gives you an array with length 5 but no values, hence you can’t iterate over it.

  • Array.apply(null, Array(5)).map(function () {}) gives you an array with length 5 and undefined as values, now it can be iterated over.

  • Array.apply(null, Array(5)).map(function (x, i) { return i; }) gives you an array with length 5 and values 0,1,2,3,4.

  • Array(5).forEach(alert) does nothing, Array.apply(null, Array(5)).forEach(alert) gives you 5 alerts

  • ES6 gives us Array.from so now you can also use Array.from(Array(5)).forEach(alert)

  • If you want to initialize with a certain value, these are good to knows…
    Array.from('abcde'), Array.from('x'.repeat(5))
    or Array.from({length: 5}, (v, i) => i) // gives [0, 1, 2, 3, 4]

More Related Contents:

  1. What is array literal notation in javascript and when should you use it?
  2. what happens when an item is deleted from a javascript array [duplicate]
  3. Concat two arrays But concat is not working
  4. Find a value in an array of objects in Javascript [duplicate]
  5. Javascript Array.sort implementation?
  6. Remove array element based on object property
  7. How to get character array from a string?
  8. How to find index of all occurrences of element in array?
  9. How to compare arrays in JavaScript?
  10. Remove duplicate values from JS array [duplicate]
  11. How can I get the index of an object by its property in JavaScript?
  12. Splitting a JS array into N arrays
  13. Is reading the `length` property of an array really that expensive an operation in JavaScript?
  14. How to combine an array in javascript
  15. Array() vs new Array()
  16. How do I check in JavaScript if a value exists at a certain array index?
  17. Should I use JSLint or JSHint JavaScript validation? [closed]
  18. How can I extend Array.prototype.push()?
  19. break array of objects into separate arrays based on a property
  20. How to check if array is empty or does not exist? [duplicate]
  21. Finding out how many times an array element appears
  22. How to check if an array is empty or exists?
  23. Recursively filter array of objects
  24. Generate permutations of JavaScript array [duplicate]
  25. How to create an array of objects from multiple arrays
  26. How to correctly use axios params with arrays
  27. Transform Javascript Array into delimited String
  28. Find closest date in array with JavaScript
  29. Sum values of objects in array
  30. Find the shortest string in array

Leave a Comment