How do I convert array of Objects into one Object in JavaScript?

Tiny ES6 solution can look like:

var arr = [{key:"11", value:"1100"},{key:"22", value:"2200"}];
var object = arr.reduce(
  (obj, item) => Object.assign(obj, { [item.key]: item.value }), {});

console.log(object)

Also, if you use object spread, than it can look like:

var object = arr.reduce((obj, item) => ({...obj, [item.key]: item.value}) ,{});

One more solution that is 99% faster is(tested on jsperf):

var object = arr.reduce((obj, item) => (obj[item.key] = item.value, obj) ,{});

Here we benefit from comma operator, it evaluates all expression before comma and returns a last one(after last comma). So we don't copy obj each time, rather assigning new property to it.

This should do it:

var array = [
    { key: 'k1', value: 'v1' },
    { key: 'k2', value: 'v2' },
    { key: 'k3', value: 'v3' }
];
var mapped = array.map(item => ({ [item.key]: item.value }) );
var newObj = Object.assign({}, ...mapped );
console.log(newObj );

var newObj = Object.assign({}, ...(array.map(item => ({ [item.key]: item.value }) )));