I’m developing an Outlook add-in that’s supposed to work with both Outlook on the Web and Outlook 365 for Windows and Mac. I’m encountering an issue with the item.body.prependAsync method. When I try to call this method, I get an error saying item.body.prependAsync is not a function.
Here’s what console.log(item.body) gives me:
{type: undefined, getAsync: ƒ}
getAsync: ƒ wt(e)
type: undefined
[[Prototype]]: Object
Here’s how I’m structuring my html:
<!DOCTYPE html>
<html>
<head>
<title>Addin test</title>
<script src="https://appsforoffice.microsoft.com/lib/1/hosted/office.js"></script>
<script>
let item;
Office.onReady((info) => {
if (info.host === Office.HostType.Outlook) {
item = Office.context.mailbox.item;
console.log(item.body);
item.body.prependAsync("text", function (asyncResult) {
if (asyncResult.status === Office.AsyncResultStatus.Failed) {
console.log("Action failed with error: " + asyncResult.error.message);
return;
}
console.log(`"${'text'}" prepended to the body.`);
});
}
});
</script>
</head>
</html>
I receive that error in my logs:
Uncaught TypeError: item.body.prependAsync is not a function
at URLtoHTML/html?et=:14:19
at appsforoffice.microsoft.com/lib/1/hosted/office.js:76:23268
at t (appsforoffice.microsoft.com/lib/1/hosted/office.js:76:22669)
at appsforoffice.microsoft.com/lib/1/hosted/office.js:76:29316
at c (appsforoffice.microsoft.com/lib/1/hosted/office.js:76:4168)
at f.waitForFunction (appsforoffice.microsoft.com/lib/1/hosted/office.js:76:4257)
at e.j [as appReadyCallback] (appsforoffice.microsoft.com/lib/1/hosted/office.js:76:28650)
at appsforoffice.microsoft.com/lib/1/hosted/outlook-web-16.01.js:20:339988
This is my slightly reduced manifest.xml:
<?xml version="1.0" encoding="UTF-8"?>
<OfficeApp
xmlns="http://schemas.microsoft.com/office/appforoffice/1.1"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:bt="http://schemas.microsoft.com/office/officeappbasictypes/1.0"
xmlns:mailappor="http://schemas.microsoft.com/office/mailappversionoverrides/1.0"
xsi:type="MailApp">
<Id>7218ed19-90f5-4ad6-99e8-8d17ee2556</Id>
<Version>1.0.0.0</Version>
<Hosts>
<Host Name="Mailbox" />
</Hosts>
<Requirements>
<Sets>
<Set Name="Mailbox" MinVersion="1.1" />
</Sets>
</Requirements>
<FormSettings>
<Form xsi:type="ItemRead">
<DesktopSettings>
<SourceLocation DefaultValue="URL to HTML" />
<RequestedHeight>250</RequestedHeight>
</DesktopSettings>
</Form>
</FormSettings>
<Permissions>ReadWriteItem</Permissions>
<Rule xsi:type="RuleCollection" Mode="Or">
<Rule xsi:type="ItemIs" ItemType="Message" FormType="Read" />
</Rule>
<DisableEntityHighlighting>false</DisableEntityHighlighting>
<VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides"
xsi:type="VersionOverridesV1_0">
<Requirements>
<bt:Sets DefaultMinVersion="1.1">
<bt:Set Name="Mailbox" />
</bt:Sets>
</Requirements>
<Hosts>
<Host xsi:type="MailHost">
<DesktopFormFactor>
<FunctionFile resid="Commands.Url" />
<ExtensionPoint xsi:type="MessageReadCommandSurface">
<OfficeTab id="TabDefault">
<Group id="msgReadGroup">
<Label resid="GroupLabel" />
<Control xsi:type="Button" id="msgReadOpenPaneButton">
<Label resid="TaskpaneButton.Label" />
<Supertip>
<Title resid="TaskpaneButton.Label" />
<Description resid="TaskpaneButton.Tooltip" />
</Supertip>
<Icon>
<bt:Image size="16" resid="Icon.16x16" />
<bt:Image size="32" resid="Icon.32x32" />
<bt:Image size="80" resid="Icon.80x80" />
</Icon>
</Control>
</Group>
</OfficeTab>
</ExtensionPoint>
</DesktopFormFactor>
</Host>
</Hosts>
<Resources>
<bt:Urls>
<bt:Url id="Commands.Url" DefaultValue="URL to HTML" />
</bt:Urls>
<bt:ShortStrings>
<bt:String id="GroupLabel" DefaultValue="Name" />
<bt:String id="TaskpaneButton.Label" DefaultValue="Action" />
</bt:ShortStrings>
</Resources>
</VersionOverrides>
</OfficeApp>
I’m testing this in Outlook on the Web at https://outlook.office365.com/. Any idea why item.body might not be the expected Body object? Any help would be greatly appreciated!

This API is only supported during compose, so try using this API with a MessageComposeCommandSurface extension point in your manifest instead.