```js RunJS="{n:'Examples/frontmatter/Toggle cssclass width-100',t:'s'}"
/*
 * processFrontMatter(file: TFile, fn: (frontmatter: any) => void): Promise<void>;
 * 
 * processFrontMatter - Developer Documentation  
 * {@link https://docs.obsidian.md/Reference/TypeScript+API/FileManager/processFrontMatter}
 * 
 * FileManager | Obsidian Plugin Developer Docs  
 * {@link https://marcus.se.net/obsidian-plugin-docs/reference/typescript/classes/FileManager#processfrontmatter}  
 */
const frontmatterName = "cssclass";
const frontMatterValue = "width-100";

const leaf = this.app.workspace.getLeaf();
this.app.workspace.setActiveLeaf(leaf);

const file = leaf.view.file;

this.app.fileManager.processFrontMatter(file, (frontMatter) => {
	let frontMatterKey = frontMatter[frontmatterName];
	if(frontMatterKey?.split(/\s+/).contains(frontMatterValue)) {
		let splits = frontMatterKey.split(/\s+/);
		splits.splice(splits.indexOf(frontMatterValue), 1);
		frontMatterKey = splits.join(" ");
	}
	else {
		if(frontMatterKey) frontMatterKey += " " + frontMatterValue;
		else frontMatterKey = frontMatterValue;
	}
	frontMatter[frontmatterName] = frontMatterKey;
});
```

```js RunJS="{n:'Examples/frontmatter/Set Now to Modified Date',t:'s'}"
const frontmatterName = "date modified";

const now = new Date();
const year = now.getFullYear();
const month = ("0" + (now.getMonth() + 1)).slice(-2);
const date = ("0" + now.getDate()).slice(-2);
const hours = ("0" + now.getHours()).slice(-2);
const minutes = ("0" + now.getMinutes()).slice(-2);
const seconds = ("0" + now.getSeconds()).slice(-2);

const frontMatterValue = year + "-" + month + "-" + date + " " + hours + ":" + minutes;

const leaf = this.app.workspace.getLeaf();
this.app.workspace.setActiveLeaf(leaf);

const file = leaf.view.file;

this.app.fileManager.processFrontMatter(file, (frontMatter) => {
	frontMatter[frontmatterName] = frontMatterValue;
});
```