How do we add field code for Word using word javascript api

I want to add field code using word JavaScript API. I checked the API documentation but I found nothing regarding this. Is there a way to do it in this API?

There is no direct support in the API for adding fields, i.e. you will have to create Open XML containing the field. You can then insert this Open XML in your document (something a long the lines of the following untested snippet):

// Run a batch operation against the Word object model.
Word.run(function (context) {

    // Queue a command to get the current selection and then
    // create a proxy range object with the results.
    var range = context.document.getSelection();

    // Queue a commmand to insert OOXML in to the beginning of the range.
    range.insertOoxml("<pkg:package xmlns:pkg='http://schemas.microsoft.com/office/2006/xmlPackage'>
         <pkg:part pkg:name='/_rels/.rels' pkg:contentType='application/vnd.openxmlformats-package.relationships+xml' pkg:padding='512'>
             <pkg:xmlData>
                 <Relationships xmlns='http://schemas.openxmlformats.org/package/2006/relationships'><Relationship Id='rId1' Type='http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument' Target='word/document.xml'/></Relationships>
             </pkg:xmlData>
         </pkg:part>
         <pkg:part pkg:name='/word/document.xml' pkg:contentType='application/vnd.openxmlformats-officedocument.wordprocessingml.document.main+xml'>
             <pkg:xmlData>
                 <w:document xmlns:w='http://schemas.openxmlformats.org/wordprocessingml/2006/main' >
                 <w:body>
                 <w:p w:rsidR="00000000" w:rsidRDefault="0043114D">
                     <w:r>
                        <w:fldChar w:fldCharType="begin"/>
                     </w:r>
                     <w:r>
                        <w:instrText xml:space="preserve"> PAGE  \* Arabic  \* MERGEFORMAT </w:instrText>
                     </w:r>
                     <w:r>
                        <w:fldChar w:fldCharType="separate"/>
                     </w:r>
                     <w:r>
                         <w:rPr>
                            <w:noProof/>
                         </w:rPr>
                         <w:t>1</w:t>
                     </w:r>
                     <w:r>
                         <w:fldChar w:fldCharType="end"/>
                     </w:r>
                     </w:p>
                 </w:body>
                 </w:document>
             </pkg:xmlData>
         </pkg:part>
    </pkg:package>", Word.InsertLocation.start);

    // Synchronize the document state by executing the queued commands,
    // and return a promise to indicate task completion.
    return context.sync().then(function () {
        console.log('OOXML added to the beginning of the range.');
    });
})
.catch(function (error) {
    console.log('Error: ' + JSON.stringify(error));
    if (error instanceof OfficeExtension.Error) {
        console.log('Debug info: ' + JSON.stringify(error.debugInfo));
    }
});