getNumberFormatId(code) {
        let id = this._numberFormatIdsByCode[code];
        if (id === undefined) {
            id = this._nextNumFormatId++;
            this._numberFormatCodesById[id] = code;
            this._numberFormatIdsByCode[code] = id;

            this._numFmtsNode.children.push({
                name: 'numFmt',
                attributes: {
                    numFmtId: id,
                    formatCode: code
                }
            });
        }

        return id;
    }
Get the nuumber format ID for a given code.
@param {string} code - The format code.
@returns {number} The number format ID.