It is usually safer to provide an initial value because there are three possible outputs without initialValue, as shown in the following example.Ĭopy Code // Production steps of ECMA-262, Edition 5, 15.4.4.21 // Reference: if (!Array.prototype. If the array has only one element (regardless of position) and no initialValue was provided, or if initialValue is provided but the array is empty, the solo value would be returned without calling callback. If the array is empty and no initialValue was provided, TypeError would be thrown. If initialValue is provided, it will start at index 0. Note: If initialValue isn't provided, reduce will execute the callback function starting at index 1, skipping the first index. If no initialValue was provided, then previousValue will be equal to the first value in the array and currentValue will be equal to the second. If initialValue is provided in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. The first time the callback is called, previousValue and currentValue can be one of two values. Reduce executes the callback function once for each element present in the array, excluding holes in the array, receiving four arguments: The reducer is provided by the programmer. The reduce function executes a reducer function on each element of the array, resulting in a single output value. The value that results from the reduction. last modified OctoJavaScript reduce tutorial shows how to use reduction operations in JavaScript language. Value to use as the first argument to the first call of the callback. Syntax: array. Starts at index 0, if an initialValue is provided, and at index 1 otherwise. The arr.reduce() method in JavaScript is used to reduce the array to a single value and executes a provided function for each value of the array (from left-to-right) and the return value of the function is stored in an accumulator. currentIndex The index of the current element being processed in the array. (See below.) currentValue The current element being processed in the array. PreviousValue The value previously returned in the last invocation of the callback, or initialValue, if supplied. reduce(callback ) Parameters callback Function to execute on each value in the array, taking four arguments:
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