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Tutorial
The For In Loop
The JavaScript for in statement loops through the properties of an Object:
Syntax:
for (key in object) {
// code block to be executed
}
Example Explained:
• The for in loop iterates over a person object
• Each iteration returns a key (x)
• The key is used to access the value of the key
• The value of the key is person[x]
EXAMPLE ❯
For In Over Arrays
The JavaScript for in statement can also loop over the properties of an Array:
Syntax:
for (variable in array) {
// code
}
Do not use for in over an Array if the index order is important.
The index order is implementation-dependent, and array values may not be accessed in the order you expect.
It is better to use a for loop, a for of loop, or Array.forEach() when the order is important.
EXAMPLE ❯
Array.forEach()
The forEach() method calls a function (a callback function) once for each array element.
Note that the function takes 3 arguments:
• The item value
• The item index
• The array itself