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The role of brackets' [] 'in the parameter writing method in MDN documents

2022-04-23 05:38:00 【Unspoken】

see MDN Documents often see the following writing ：

    Array.from(arrayLike[, mapFn[, thisArg]])

Simply speaking , Brackets indicate that a parameter is not required , Above all ：

    [, mapFn] It indicates that the mapFn Optional 
    [, thisArg] It indicates that the thisArg Optional 
    [, mapFn[, thisArg]] said thisArg It depends on mapFn There is , If mapFn There is no passing value , that thisArg It cannot be passed in .