# JavaScript: Apply a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values

## JavaScript: Exercise-156 with Solution

Write a JavaScript program to apply a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values.

• Use Array.prototype.reduce() to apply the given function to the given array, storing each new result.

Sample Solution:

JavaScript Code:

``````//#Source https://bit.ly/2neWfJ2
const reduceSuccessive = (arr, fn, acc) =>
arr.reduce((res, val, i, arr) => (res.push(fn(res.slice(-1)[0], val, i, arr)), res), [acc]);

console.log(reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0));
```
```

Sample Output:

```[0,1,3,6,10,15,21]
```

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-156-1 by w3resource (@w3resource) on CodePen.

Improve this sample solution and post your code through Disqus

What is the difficulty level of this exercise?

Test your Programming skills with w3resource's quiz.

﻿

## JavaScript: Tips of the Day

Checking if a key exists in a JavaScript object?

Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?

```var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!
```

You should instead use the in operator:

`"key" in obj // true, regardless of the actual value`

If you want to check if a key doesn't exist, remember to use parenthesis:

```!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"
```

Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:

```obj.hasOwnProperty("key") // true
```

Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined? var obj = { key: undefined }; obj["key"] !== undefined // false, but the key exists! You should instead use the in operator: "key" in obj // true, regardless of the actual value If you want to check if a key doesn't exist, remember to use parenthesis: !("key" in obj) // true if "key" doesn't exist in object !"key" in obj // ERROR! Equivalent to "false in obj" Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty: obj.hasOwnProperty("key") // true For performance comparison between the methods that are in, hasOwnProperty and key is undefined.

Ref: https://bit.ly/2CFNp1X