How to Build a Custom Promise.all Polyfill
How to implement async batch processing operations by building a manual Promise.all constructor wrapper.
Understand Promise.all Requirements
Identify that Promise.all takes an iterable collection of promises and returns a single promise that resolves only when all input promises resolve, or rejects instantly if any promise fails.
Declare the Core Return Wrapper
Attach a custom method to the global object: 'Promise.myAll = function(promises) { return new Promise((resolve, reject) => { ... }) }'.
Initialize Result and Counter Variables
Inside the returned promise execution context block, declare an empty array 'resolvedValues' and a numerical counter 'completedCount' set to 0.
Loop Through the Input Collection
Iterate through the array of input promises using a standard loop or a '.forEach()' array iteration routine.
Normalize Values with Promise.resolve
Wrap each element in 'Promise.resolve(promises[i])' to ensure primitive values are converted into proper promises automatically.
Capture Resolved Element Outputs
Attach a '.then()' success callback handler to each promise instance in the loop.
Save Elements by Exact Array Index
Inside the success callback, save the resolved output directly into the 'resolvedValues' array at the matching index 'i': 'resolvedValues[i] = value'.
Increment and Resolve the Promise
Increment your 'completedCount' by 1 after each success. Check if 'completedCount === promises.length'; if true, call 'resolve(resolvedValues)' to resolve the main promise.
Implement Immediate Rejection Traps
Attach a '.catch(reject)' failure callback handler directly to each promise in the loop. If any individual promise fails, the main wrapper breaks and rejects instantly.
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