test.js
const testList = [1, 2, 2, 4, 5, 2, 4, 2, 4, 5, 5, 6, 7, 7, 8, 8, 8, 1, 4, 1, 1];
const lastIndex = testList.findLastIndex((e:number) => e === 100);
// Property 'findLastIndex' does not exist on type 'number[]'. Did you mean 'findIndex'?ts(2551)
// lib.es2015.core.d.ts(43, 5): 'findIndex' is declared here.
tsconfig.json
...
"target": "ES6",
...
Are there any specific ts settings required for the findLastIndex method?
The array methods findLast() and findLastIndex() are currently part of a proposal that has advanced to Stage 4 in the TC39 process as of January 15, 2023. Although they are not yet in any released version of ECMAScript, they are included in the upcoming ESNext version and will be available in ES2023.
In the case of TypeScript, support for these methods was requested through microsoft/TypeScript#48829. The additions have been implemented in microsoft/TypeScript#49636, which was recently merged into the main branch on January 13, 2023. While no current version of TypeScript includes these methods, they are present in the nightly build typescript@next and will be officially released with TypeScript 5.0.
To utilize these methods without defining them yourself, it is recommended to use the nightly version of TypeScript with the --target set to either "ESNext" or potentially "ES2023" if applicable.
For those interested, here's a Playground link to code using TS5.0-dev with ESNext target
let lastPosition = -1;
for (let j = myList.length - 1; j >= 0; j--) {
if (myList[j] === 100) {
lastPosition = j;
break;
}
}
console.log(lastPosition); // -1 if 100 is not found, otherwise the last position of 100
It appears that the issue you are experiencing is related to TypeScript. You may find some helpful discussions on this topic:
Based on my understanding, this particular answer may provide the solution you are seeking:
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