checked false

1   <input type="checkbox" checked={props.completed} /> 
2
3#for props completed is true/false , hence if completed is false then checkbox will not be clicked .
4# if the checkbox is true it will be will be checked
5

1There are no values that will cause the checkbox to be unchecked. 
2If the checked attribute exists, the checkbox will be checked regardless of
3what value you set it to.
4
5<input type="checkbox" checked />
6<input type="checkbox" checked="" />
7<input type="checkbox" checked="checked" />
8<input type="checkbox" checked="unchecked" />
9<input type="checkbox" checked="true" />
10<input type="checkbox" checked="false" />
11<input type="checkbox" checked="on" />
12<input type="checkbox" checked="off" />
13<input type="checkbox" checked="1" />
14<input type="checkbox" checked="0" />
15<input type="checkbox" checked="yes" />
16<input type="checkbox" checked="no" />
17<input type="checkbox" checked="y" />
18<input type="checkbox" checked="n" />