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Javascript, map array from index

How is it possible to map an array from a index and then map the rest based on a criteria.

const array = [4, 2, 8, 3, 4, 6];

const map = array.map((x, index) => {
  if(x<8){
    return index;
  };
 
});

console.log(">> "+map);

// Output array should be [1,3,4,5];  * Final array will be a lot bigger;

For example I want to search the array from index 1 and then return the indexes of all values less than 8.

Is map the best way or is there another?

Any help / advice would be appreciated.

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2

Answers


  1. 0

    You can filter out the undefined values that are returned for the elements that don’t match the condition.

    const array = [4, 2, 8, 3, 4, 6];

const result = array.map((x, index) => {
  if(x<8){
    return index;
  };
}).filter(el => el !== undefined);

console.log(">> "+result);

    Or do it in one loop using reduce.

    const array = [4, 2, 8, 3, 4, 6];

const result = array.reduce((acc, x, index) => {
  if(x<8){
    acc.push(index);
  };
  return acc;
}, []);

console.log(">> "+result);
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  2. 0

    If you want to use an Array method like .map() wherein the return value is transformed (ex. current number to current index) but return nothing when a criteria is not met (ie filtering), try .flatMap(). .flatMap() is .map() but it will flatten the returned array (see .flat()). The example below uses a ternary operator for the the criteria which means:

    /* If the current index (i) is greater than 
OR equal to the given number (start = 1) ... */
 index >= start 
// AND...
 &&
// the current value (x) is less than 8...
 x < 8
// return the current index (i).
 ? i
/* Otherwise return an empty array 
(which will be flattened to nothing).*/
 : []

    const array = [4, 2, 8, 3, 4, 6];
const start = 1;

const result = array.flatMap((x, i) => i >= start && x < 8 ? i : []);

console.log(">> "+result);

/** 
 * Output array should be [1,3,4,5];  
 * Final array will be NOT be bigger because the callback function returns 
 * just an index number OR nothing at all for a given number value
 */
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