combine.js•1.8 kB
'use strict';
const format = require('./format');
/*
* function cascade(formats)
* Returns a function that invokes the `._format` function in-order
* for the specified set of `formats`. In this manner we say that Formats
* are "pipe-like", but not a pure pumpify implementation. Since there is no back
* pressure we can remove all of the "readable" plumbing in Node streams.
*/
function cascade(formats) {
if (!formats.every(isValidFormat)) {
return;
}
return info => {
let obj = info;
for (let i = 0; i < formats.length; i++) {
obj = formats[i].transform(obj, formats[i].options);
if (!obj) {
return false;
}
}
return obj;
};
}
/*
* function isValidFormat(format)
* If the format does not define a `transform` function throw an error
* with more detailed usage.
*/
function isValidFormat(fmt) {
if (typeof fmt.transform !== 'function') {
throw new Error([
'No transform function found on format. Did you create a format instance?',
'const myFormat = format(formatFn);',
'const instance = myFormat();'
].join('\n'));
}
return true;
}
/*
* function combine (info)
* Returns a new instance of the combine Format which combines the specified
* formats into a new format. This is similar to a pipe-chain in transform streams.
* We choose to combine the prototypes this way because there is no back pressure in
* an in-memory transform chain.
*/
module.exports = (...formats) => {
const combinedFormat = format(cascade(formats));
const instance = combinedFormat();
instance.Format = combinedFormat.Format;
return instance;
};
//
// Export the cascade method for use in cli and other
// combined formats that should not be assumed to be
// singletons.
//
module.exports.cascade = cascade;