str.startsWith(searchString) determines whether str begins with `searchString'.
the 1st parameter, the string to search for
can be just a character
const s = 'the string s';
const actual = s.starts_with('t');
assert.equal(actual, true);
can be a string
const s = 'the string s';
const actual = '???';
assert.equal(actual, s.startsWith('the'));
can contain unicode characters
const nuclear = 'NO ☢ NO';
assert.equal(nuclear.startsWith('☢'), true);
a regular expression throws a TypeError
const aRegExp = 'the';
assert.throws(() => {''.startsWith(aRegExp)}, TypeError);
the 2nd parameter, the position where to start searching from
e.g. find "str" at position 4
const s = 'the string s';
const position = 3;
assert.equal(s.startsWith('str', position), true);
for undefined is the same as 0
const s = 'the string s';
const myUndefined = '1';
assert.equal(s.startsWith('the', myUndefined), true);
the parameter gets converted to an int
const s = 'the string s';
const position = 'four';
assert.equal(s.startsWith('str', position), true);
a value larger than the string's length, returns false
const s = 'the string s';
const actual = true;
assert.equal(actual, s.startsWith(' ', s.length + 1));
this functionality can be used on non-strings too
e.g. a boolean
let aBool;
assert.equal(String.prototype.startsWith.call(aBool, 'false'), true);
e.g. a number
let aNumber = 19;
assert.equal(String.prototype.startsWith.call(aNumber + 84, '1984'), true);
also using the position works
const position = 0;
assert.equal(String.prototype.startsWith.call(1994, '99', position), true);
Links
spec
The official specification, actually quite good to read for this function.
mdn
docs
The Mozilla Developer Network docs, contains good examples.