function key2ind( key ) {
			var i;
			if ( this !== tuple ) { // eslint-disable-line no-invalid-this
				throw new TypeError( 'invalid invocation. `this` is not host tuple.' );
			}
			if ( !isString( key ) ) {
				throw new TypeError( format( 'invalid argument. First argument must be a string. Value: `%s`.', key ) );
			}
			for ( i = 0; i < nfields; i++ ) {
				if ( fields[ indices[i] ] === key ) {
					return i;
				}
			}
			return -1;
		}
Converts a field name to a tuple index.

## Notes

-   If provided an unknown field name, the function returns `-1`.

@private
@memberof tuple
@param {string} key - field name
@throws {TypeError} `this` must be the host tuple
@throws {TypeError} first argument must be a string
@returns {integer} tuple index