Since ES6 you can now use the built in
includes method eg
"testing".includes("test"); // true
One thing to note is this is case sensitive so the below will be false.
"Testing".includes("test"); // false
There is also an optional position param which defaults to 0.
For example the below will return true.
"sand".includes("and"); "sand".includes("and", 1);
However this will return false, because we are only including the part of the string after charecter 2 eg
For more information be sure to read about includes on the MDN website.