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this thread
/**
* @type {Array<String>}
*/
const arr = ['one', 'two', 'three'];
/** @type {Array<[string, number]>} */
const init = [];
/**
* @type {Array<[string, number]>}
*/
const result = arr.reduce((acc, item) => {
acc.push([item, item.length]);
return acc;
}, init);