how to check the checked attribute value in javascript code example

Example 1: how to make checkbox checked in html

<input type="checkbox" checked={props.completed} /> 

#for props completed is true/false , hence if completed is false then checkbox will not be clicked .
# if the checkbox is true it will be will be checked

Example 2: javascript get boolean if checkbox is checked

var checkbox = document.getElementById('checkbox');

if(checkbox.checked){
    console.log('test');
}