This piece of PowerShell code helps to setup authentication info to Azure Artifact npm feed.

Auth-AzNpmFeed.ps1

[CmdletBinding()]
param (
    [String]$Registry,
    [SecureString]$Token
)

$SampleRegistry = 'For example: //pkgs.dev.azure.com/project/npm/registry/'

function Validate-Registry {
    param (
        [String]$Registry
    )

    if (-Not($Registry.StartsWith('//'))) {
        Write-Error "Registry config must start with //. `n$sampleRegistry" -ErrorAction Stop
    }

    If (-Not($Registry.EndsWith('/'))) {
        Write-Error "Registry config must end with /. `n$sampleRegistry" -ErrorAction Stop
    }
}

function Encode-ToBase64 {
    param (
        [SecureString]$Token
    )

    if ($PSVersionTable.PSVersion.Major -gt 5) {
        # PowerShell Core 7's method
        $bytes = [System.Text.Encoding]::ASCII.GetBytes($($Token | ConvertFrom-SecureString -AsPlainText))
        $encodedToken = [Convert]::ToBase64String($bytes) | ConvertTo-SecureString -AsPlainText
    }
    else {
        # PowerShell 5's method
        $bytes = [System.Text.Encoding]::ASCII.GetBytes($(New-Object PSCredential "user", $Token).GetNetworkCredential().Password)
        $encodedToken = [Convert]::ToBase64String($bytes) | ConvertTo-SecureString -AsPlainText -Force
    }
    $encodedToken
}

function Run-NpmCommands {
    param (
        [String]$Registry,
        [SecureString]$Base64Token
    )

    if (!(Get-Command npm)) {
        Write-Error "npm command line is required to proceed. Please install npm cli: https://nodejs.org/en/download/" -ErrorAction Stop
    }

    npm config set "$Registry`:username"   "bearer"
    $OverallExitCode1 = 0 -bor $LASTEXITCODE
    npm config set "$Registry`:email"      "no-reply@one.com"
    $OverallExitCode2 = $OverallExitCode1 -bor $LASTEXITCODE

    if ($PSVersionTable.PSVersion.Major -gt 5) {
        # PowerShell Core 7's method
        npm config set "$Registry`:_password"  $($Base64Token | ConvertFrom-SecureString -AsPlainText)
    }
    else {
        # PowerShell 5's method
        npm config set "$Registry`:_password"  $(New-Object PSCredential "user", $Base64Token).GetNetworkCredential().Password
    }

    $OverallExitCode3 = $OverallExitCode2 -bor $LASTEXITCODE

    if ($OverallExitCode3 -ne 0) {
        Write-Error "npm config set commands did not exit correctly." -ErrorAction Stop
    }
}


if (!$Registry) {
    $RegistryInput = Read-Host -Prompt "Enter registry config. $SampleRegistry"
}
else {
    $RegistryInput = $Registry
}

Validate-Registry -Registry $RegistryInput

if (!$Token) {
    $TokenInput = Read-Host -Prompt "Enter non-base64 encoded token" -AsSecureString
}

$Base64SecureToken = Encode-ToBase64 -Token $TokenInput

Run-NpmCommands -Registry $RegistryInput -Base64Token $Base64SecureToken