# JavaScript: Get the minimum value of an array, after mapping each element to a value using the provided function

## JavaScript fundamental (ES6 Syntax): Exercise-177 with Solution

Write a JavaScript program to get the minimum value of an array, after mapping each element to a value using the provided function.

• Use Array.prototype.map() to map each element to the value returned by fn.
• Use Math.min() to get the minimum value.

Sample Solution:

JavaScript Code:

``````//#Source https://bit.ly/2neWfJ2
const minBy = (arr, fn) => Math.min(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
console.log(minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n));
console.log(minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'));
```
```

Sample Output:

```2
2
```

Pictorial Presentation:

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-177-1 by w3resource (@w3resource) on CodePen.

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## 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