程序化使用

The main entry point into less is the less.render function. This takes the following format

```less
less.render(lessInput, options)
    .then(function(output) {
        // output.css = string of css
        // output.map = string of sourcemap
        // output.imports = array of string filenames of the imports referenced
    },
    function(error) {
    });

// or...

less.render(css, options, function(error, output) {})
```
The options argument is optional. If you specify a callback then a promise will not be returned, where as if you do not specify a callback a promise will be given. Under the hood, the callback version is used so that less can be used synchronously.

If you wanted to render a file, you would first read it into a string (to pass to less.render) and then set the filename field on options to be the filename of the main file. less will handle all the processing of the imports.

The `sourceMap` option is an object which enables you to set sub sourcemap options. Available sub options are: `sourceMapURL`,`sourceMapBasepath`,`sourceMapRootpath`,`outputSourceFiles` and `sourceMapFileInline`. Notice that the sourceMap option is not available for the less.js in browser compiler now.

```less
less.render(lessInput)
    .then(function(output) {
        // output.css = string of css
        // output.map = undefined
}
//,
less.render(lessInput, {sourceMap: {}})
    .then(function(output) {
        // output.css = string of css
        // output.map = string of sourcemap
}
//or,
less.render(lessInput, {sourceMap: {sourceMapFileInline: true}})
    .then(function(output) {
        // output.css = string of css \n /*# sourceMappingURL=data:application/json;base64,eyJ2ZXJ..= */
        // output.map = undefined
}
```
### Getting Access to the Log

You can add a log listener with the following code

```less
less.logger.addListener({
    debug: function(msg) {
    },
    info: function(msg) {
    },
    warn: function(msg) {
    },
    error: function(msg) {
    }
});
```
**Note:** all functions are optional. An error will not be logged, but instead is passed back to the callback or promise in less.render