Loops: For Loops, While Loops, For...Of Loops, For...In Loops-JS Tutorial-php.cn

Loops: For Loops, While Loops, For...Of Loops, For...In Loops

The point of looping is to repeat some functionality.

Some types of loops include:

for loop
while loop
for...of loop
for...in loop

For Loop

To can write a simple for loop as follows:

for (let i = 1; i <= 10; i++) { console.log(i); // prints numbers 1-10 }

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To loop over an array, we can do as follows:

const animals = ['lizard', 'fish', 'turtle']; for (let i = 0; i < animals.length; i++) { console.log(i, animals[i]); } // 0 'lizard' // 1 'fish' // 2 'turtle'

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We can also loop over this array in reverse:

for (let i = animals.length - 1; i >= 0; i--) { console.log(i, animals[i]); } // 2 'turtle' // 1 'fish' // 0 'lizard'

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We can also make a loop within a loop:

for (let i = 0; i <= 2; i++) { for (let j = 0; j < 2; j++) { console.log(`i=${i}, j=${j}`); } } // i=0, j=0 // i=0, j=1 // i=1, j=0 // i=1, j=1 // i=2, j=0 // i=2, j=1

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This is useful if we want to iterate over an array of arrays:

const seatingChart = [ ['Abigale', 'Tim', 'Cynthia'], ['Bob', 'Carter', 'Zane', 'Tanja'], ['Quin', 'Xavier'], ]; // To print each name individually from seatingChart: for (let i = 0; i < seatingChart.length; i++) { for (let j = 0; j < seatingChart[i].length; j++) { console.log(seatingChart[i][j]); } }

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While Loop

An example of a simple while loop is:

let num = 0; // to print out 0 through 4: while (num < 5) { console.log(num); num++; }

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Break keyword

The break keyword can be used to exit a while loop:

let input = prompt('Say something:'); while (true) { input = prompt(input); if (input === 'stop copying me') { break; // finally stops prompting user } }

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It can also be used to exit a for loop. Let's say we have the line:

let line = ['Abby', 'Salvia', 'Jamie', 'Carter', 'John'];

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and we want to output everyone that comes before Jamie, but not Jamie:

for (let i = 0; i < line.length; i++) { if (line[i] === 'Jamie') break; console.log(line[i]); }

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For...Of Loops

If we want to print each value from an array, we can do something like:

let people = ['Agitha', 'Bruce', 'Charlie', 'Dane', 'Ernie']; // to print each persons name: for (let person of people) { console.log(person); }

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To make our seating chart example from before more legible, we can do:

const seatingChart = [ ['Abigale', 'Tim', 'Cynthia'], ['Bob', 'Carter', 'Zane', 'Tanja'], ['Quin', 'Xavier'], ]; // To print each name individually from seatingChart: for (let row of seatingChart) { for (let person of row) { console.log(person); } }

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For...In Loops

If we want to iterate through each key value pair in an object, we can do something like:

const testScores = { jim: 34, abby: 93, greg: 84, mark: 95, melvin: 73, }; for (let person in testScores) { console.log(`${person} scored ${testScores[person]}`); }

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If we want to get the average of the testScores using For...Of, we can do as follows:

let total = 0; let scores = Object.values(testScores); for (let score of scores) { total += score; } let avg = total / scores.length; console.log(avg);

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