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Pablo Ezequiel Inchausti
Asked: 2020-07-31 10:39:39 +0800 CST 2020-07-31 10:39:39 +0800 CST

How to do a break inside an array.forEach in JavaScript?

  • 772

How can you make a breakof a Array.forEachin JavaScript? When using a "for" loop in JavaScript the breakis used to break the loop.

From ECMA 5.1appears the possibility of using Array.forEachto iterate over array, but how can you do a breakin this type of cycles?

Let's see two examples, where the attempt to interrupt the Array.forEach loop with "return"does not give the expected result:

const firstElems_LenOne_V1 = (arr) => {
  let rta = []
  for (let i=0; i<arr.length;i++) {
    let e=arr[i] 
    if (e.length>1) break 
    rta.push(e)
  }
  return rta
}

const firstElems_LenOne_V2 = (arr) => {
  let rta = []
  arr.forEach(e=>{
    if (e.length==1) return  // break forEach?
    rta.push(e)
  })
  return rta
}

let v1= firstElems_LenOne_V1(["a","b","cc","d"]) 
let v2= firstElems_LenOne_V2(["a","b","cc","d"]) 

console.log(v1)  // OK: expect [a,b] got [a,b]
console.log(v2)  // KO: expect [a,b] got [cc]

How to do a breakinside an array.forEach in Javascript?

javascript

2 Answers

  1. Best Answer

    The forEach documentation has a note that there is no way to stop or kill a forEachunless it is a Exception, if you want to do this the method forEachwould not be appropriate. Instead they recommend using some() if you need to return a value boleanoor just a simple iteration as in this case.

    custom exception

    function BreakException(message) {
   this.message = message;
   this.name = 'Exception';
}

const firstElems_LenOne_V2 = (arr) => {
    let rta = []
    try {
       arr.forEach(e=>{
        if (e.length===1) throw new BreakException('Valor ' + e + ' con Length diferente a 1');
        rta.push(e)
      })
    } catch (e) {
       console.log(e.message); 
    }
    return rta
}
let v1= firstElems_LenOne_V2(["aa","b","cc","d"])
console.log(v1);

    • 3

  2. The method Array.forEachdoes not provide a system to stop the iteration. However you can use the function Array.somewhere you can return falseto do just what you want.

    Example:

    ['a', 'b', 'c', 'd' ,'e', 'f'].some(function(item, index){
    console.log(index, item)
    return index === 3;
});

    Result:

    0 "a"
1 "b"
2 "c"
3 "d"
    • 3